Typically a furnace will have a service switch mounted on the side of the furnace or nearby wall. It should also have a dedicated breaker, possibly two. There's no need to shutoff the entire house, just the HVAC system itself.
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BMitch♦May 20 '13 at 22:48

Sounds like CYA Lawyerspeak for a high dollar consumer device that they know will be installed by people with absolutely no experience working on electrical and HVAC systems. It's easier and causes the least liability to tell the consumer to throw the master switch for all power than try to tell them how to find the circuit breaker for the HVAC system, and possibly any ancilliary breaker to power secondary devices. All that's needed is to shut down the HVAC and make sure that it is the same circuit the 24v control voltage comes from. Once again, consumers got no VOMs, assume the worst.
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Fiasco LabsMay 21 '13 at 0:33

4 Answers
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I've not installed a Nest thermostat, but I have installed a BAYweb thermostat (similar concept). The precaution is with the HVAC systems that are being controlled with the thermostat, not the entire house.

In this case, safely turning off anything being controlled by the thermostat should suffice. And to be on the safe side, flip their breakers as well.

Some systems are line-voltage in which case you could get a 120V or even 240V shock from the thermostat wiring. I believe the Nest is designed for 24V systems. It does seem a bit extreme to suggest turning off the power to your entire house. I would probably settle with turning off the power to your furnace and then verifying with a non-contact voltage tester to ensure none of the wires are live.

On many thermostats, you can verify they are not getting power from the furnace by removing the batteries - if this display stays on, it's getting external power. If the display dies, then its not.

Shutting off the house seems extreme, but you could easily damage the furnace control board if you let the wrong wires touch during the installation. Most furnaces have a fuse to protect the 24V components, though.